refactor: persist explicit 'light' theme preference instead of removing storage key

- Change toggleTheme to store 'light' in localStorage instead of removing the key when switching to light mode
- Ensures theme preference is always explicitly stored rather than relying on key absence

Author: gustavotoyota

new-deepnotes/apps/web/src/features/theme/useThemePreference.ts

@@ -39,7 +39,7 @@ export function toggleTheme(): void {
     localStorage.setItem(STORAGE_KEY, "dark");
   } else {
     html.classList.remove("dark");
-    localStorage.removeItem(STORAGE_KEY);
+    localStorage.setItem(STORAGE_KEY, "light");
   }
   _isDark.value = next;
 }